1. Field of the Invention
The present invention relates to a control apparatus for a hydraulically operated transmission mainly used as a hydraulically operated vehicular transmission.
2. Description of Related Art
As hydraulically operated transmissions, there are conventionally known a parallel-2-shaft type of transmission which is made up of parallely disposed input and output shafts and a plurality of gear trains having interposed a hydraulic clutch in each of them, and a planetary gear type of transmission which is made up of a combination of a planetary gear mechanism, hydraulic clutches and hydraulic brakes. They are both arranged to perform speed changing among a plurality of transmission trains by using the hydraulic engaging elements which are made up of hydraulic clutches, hydraulic brakes, or the like.
Conventionally, as a control apparatus for this kind of hydraulically operated transmission, there is known one in which a plurality of solenoid valves corresponding in number to the plurality of hydraulic engaging elements are provided so that the hydraulic oil supply to, and discharge from, each of the hydraulic engaging elements can be controlled. In this control apparatus, there is the following problem. Namely, when the solenoid valves are poorly operated or wrongly operated, hydraulic engaging elements other than that for establishing a desired transmission train are also engaged, with the result that there occurs a so-called simultaneous engagement in which a plurality of transmission trains are simultaneously established.
In order to solve this kind of problem, there is known an apparatus in Japanese Published Unexamined Patent Application No. 3727/1990. In this apparatus, between a solenoid valve and a hydraulic engaging element, there is provided a changeover valve which can be changed over between an open position in which the solenoid valve and the hydraulic engaging element are connected together and a closed position in which this connection is shut off. Upon this changeover valve there are operated a hydraulic oil pressure from another hydraulic engaging element and a hydraulic oil pressure from the solenoid valve as a hydraulic oil pressure to urge the changeover valve to the closed position. It is thus so arranged that the changeover valve is changed over to the closed position if the oil pressure from the solenoid valve rises due to a wrong operation at the time of establishing another transmission train, whereby the engagement of the hydraulic engaging element on the downstream side of the solenoid valve is blocked.
Further, from the Japanese Published Unexamined Patent Application No. 60271/1992, there is known the following apparatus. Namely, in this apparatus a changeover valve is interposed between a solenoid valve for controlling the hydraulic oil supply to, and discharge from, a hydraulic engaging element and an oil supply passage on an upstream side of the solenoid valve. To this changeover valve there is inputted, as a hydraulic oil pressure for urging the changeover valve towards a closed position, a hydraulic oil pressure of another hydraulic engaging element. At the time of engagement of said another hydraulic engaging element, the changeover valve is changed over to the closed position to thereby stop the oil supply to the solenoid valve. The engagement of the hydraulic engaging element on the downstream side of the solenoid valve is thus blocked.
In the apparatus like the above-described former conventional example in which the changeover valve is provided on the downstream side of the solenoid valve, the length of the oil passage between the solenoid valve and the hydraulic engaging element becomes long, resulting in a larger flow resistance in the oil passage. It follows that there will occur a delay in the hydraulic oil supply to, and discharge from, the hydraulic engaging element by the closing and opening of the solenoid valve, resulting in a poor response in the speed changing.
On the other hand, if the changeover valve is provided on the upstream side of the solenoid valve as in the above-described letter conventional example, the length of the oil passage between the solenoid valve and the hydraulic engaging element can be shortened. However, when the speed changing is performed by releasing the engagement of said another hydraulic engaging element and also by engaging the hydraulic engaging element on the downstream side of the solenoid valve, the changeover valve is held in the closed position until the hydraulic oil pressure in said another hydraulic engaging element is lowered below a predetermined value. Therefore, the hydraulic oil cannot be supplied to the hydraulic engaging element on the downstream side of the solenoid valve and, consequently, the starting of the oil supply is delayed. This results in a poor response in the speed changing like in the above-described former conventional example.
In this case, the following may be considered. Namely, to the changeover valve there is also inputted the hydraulic oil pressure of the hydraulic engaging element on the downstream side of the solenoid valve as the hydraulic oil pressure for urging the changeover valve towards the closed position. The changeover valve is then changed over to the closed position at the time of a failure when the hydraulic oil pressure on the downstream side of the solenoid valve rises by a wrong operation at the time of engaging said another hydraulic engaging element. At the time of speed changing, the oil supply to the hydraulic engaging element on the downstream side of the solenoid valve is then arranged to be started from the beginning. In this arrangement, however, the changeover valve may once be changed over to the closed position at the time of a failure, but the hydraulic oil pressure in the hydraulic engaging element on the downstream side of the solenoid valve is lowered by this changing over. Consequently, the solenoid valve is opened again and the chattering of the changeover valve may occur.
In view of the above-described points, the present invention has an object of providing a control apparatus which can positively block the simultaneous engagement without impairing the response in the speed changing.